The invention relates to unlocking the potential of renewable biomass, for the production of economically valuable functional chemicals. One such chemical is furan-2,5-dicarboxylic acid (FDCA). FDCA is useful, inter alia, as a building block for polymers. In fact, it potentially serves as an alternative for terephthalic acid in the production of polyesters. FDCA based polyesters, including PEF, poly(ethylene furanoate), have been shown to be highly comparable to their terephthalate based analogues.
Although historically FDCA was first synthesized from galactaric acid, the far majority of current research efforts focuses on the use of hexoses (i.e. D-glucose and D-fructose) as starting materials and 5-hydroxymethyl-2-furaldehyde (HMF) as an intermediate (Scheme 1).

Over the past decades, many routes towards HMF have been explored, the far majority of these studies relating to glucose and fructose as starting components. Besides the routes depicted in scheme one, literature mentions failed attempts to obtain more than minute amounts of HMF by acid catalyzed dehydration of uronic acids like galacturonic or glucuronic acid.
A single publication (Votocek 1934) mentions the dehydration of a keto acid derived from sorbose, viz. by oxidation of sorbose (neutral sugar) into 5-ketogluconic acid. The highest yields of HMF are generally obtained starting from D-fructose, which is generally produced from starch containing food crops, but can also be obtained from sources like sucrose or inulin. Therefore, whereas this first generation HMF may contribute to the reduction of greenhouse gas emissions and non-renewable energy consumption, there is a potential conflict with food production.
Hence it would be desirable to develop a route to FDCA based on the use of non-food lignocellulosic feed-stocks like wood or grasses, or even more preferably from residues from agro-food production or from non-terrestrial biomass sources, in which case there is no competition with current land use.
One such abundantly available type of residues are pectins; (hetero)polysaccharides which are found in large amounts in sugar beet and chicory pulp and fruit peals. Pectins can be hydrolyzed into the free sugar monomers like galacturonic acid, arabinose, glucose, galactose, rhamnose, xylose, and efforts are being undertaken to scale up the process to an economically viable industrial scale. In this way, a significant stream of non-edible uronic acids, i.e. D-galacturonic acid, or D-glucuronic acid becomes available, which can serve as a starting material for non-food applications, such as synthetic polymers. Seaweeds, like brown seaweeds of numerous types, can also serve as a source in order to obtain uronic acids like D-mannuronic acid and L-guluronic acid, by hydrolysis of the alginate fraction of seaweeds.
A method for preparing FDCA from renewable sources such as alginate is described in WO 2013/049711. Herein 5-formyl-2-furan dicarboxylic acid (FFA) is oxidized to produce FDCA. The FFA is obtained from a specific intermediate that satisfies the following formula:

This compound is either 4-deoxy-L-erythro-5-hexoseulose (or hexosulose) urinate (DEHU) or 4-deoxy-L-threo-5-hexosulose urinate (DTHU). Accordingly, for practicing the method disclosed in WO 2013/049711, it is required that the renewable source, viz. the alginate, is subjected to a specific enzymatic cleavage method resulting in DEHU or DTHU. Therewith the method does not provide a straightforward way of valorizing the biomass concerned. Also, from a chemical point of view the intermediate, particularly by having a keto group adjacent to a carboxylic acid group, runs a risk of being unstable.
It is now desired to provide a method for the production of FDCA that commences with starting materials that, if desired, are abundantly available from bio-based sources, viz. the aforementioned uronic acids.
A problem with uronic acids, such as galacturonic acid, is that they are not stable under strong acidic conditions, as these lead to decarboxylation reactions. As a background in this respect, reference is made to Stutz et al., Helvetica Chimica Acta Volumen XXXIX, Fasciculus VII (1956), No. 245, 2126-2130. Therein it is mentioned that several degradation products result when hexuronic acids are treated with concentrated sulfuric acid. In the paper, FFA is resolved as being one of the degradation products resulting from treating D-galacturonic acid with concentrated sulfuric acid. The yield of FFA hereby is very low, viz. 25 mg FFA on the basis of 10 g galacturonic acid.
In a co-pending patent application [unpublished EP application number 12163081], we have shown that uronic acids, particularly galacturonic acid can serve as starting material for the synthesis of FDCA if conducted according to the above Scheme 1. In the first step, galacturonic acid is converted to galactaric acid (also known as mucic acid) by using a mild catalytic oxidation. This reaction proceeds fast and very selectively in aqueous media at room temperature, requiring only air as the oxidant. The subsequent cyclodehydration step however is prone to have only moderate yields (<50 mole %). In order to become economically viable, this second step needs to be significantly improved.
Therefore, an alternative synthesis strategy is desired to be able to obtain FDCA from uronic acids.